The present invention relates generally to data transmission in mobile communication systems and more specifically to methods for Hybrid Automatic Repeat Request (HARQ) process management for carrier aggregation.
As used herein, the terms “user agent” and “UA” can refer to wireless devices such as mobile telephones, personal digital assistants, handheld or laptop computers, and similar devices or other User Equipment (“UE”) that have telecommunications capabilities. In some embodiments, a UA may refer to a mobile, wireless device. The term “UA” may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). Additional improvements to LTE systems/equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the term “access device” will refer to any component, such as a traditional base station or an LTE or LTE-A access device (including eNBs), that can provide a UA with access to other components in a telecommunications system.
In mobile communication systems such as the E-UTRAN, an access device provides radio access to one or more UAs. The access device comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UAs communicating to the access device. The functions of the scheduler include, among others, dividing the available air interface capacity between UAs, deciding the transport channel to be used for each UA's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared CHannel (PDSCH) and Physical Uplink Shared CHannel (PUSCH) data transmissions, and sends scheduling information to the UAs through a scheduling channel.
Several different data control information (DCI) message formats are used by LTE access devices to communicate data packet resource assignments to UAs including, among others, DCI formats 1 and 1A. An access device selects one of the DCI formats for allocating resources to a UA as a function of several factors including UA and access device capabilities, the amount of data to transmit to/receive from a UA, the amount of communication traffic within a cell, channel conditions, etc. UAs refer to the scheduling/resource allocation information for the timing and the data rate of uplink and downlink transmissions and transmit or receive data packets accordingly. In LTE, DCI formatted control data packets are transmitted via the Physical Downlink Control CHannel (PDCCH).
Hybrid Automatic Repeat reQuest (HARQ) is a scheme for re-transmitting a traffic data packet to compensate for an incorrectly received traffic packet. A HARQ scheme is used both in uplink and downlink transmissions in LTE systems. Take downlink transmissions for example, for each downlink packet received by a UA, a positive acknowledgment (ACK) is transmitted on a Physical Uplink Control Channel (PUCCH) from the UA to the access device after a cyclic redundancy check (CRC) performed by the UA indicates a successful decoding. If the CRC indicates a packet is not received correctly, a UA HARQ entity transmits a negative acknowledgement (NACK) on the PUCCH in order to request a retransmission of the erroneously received packet. Once a HARQ NACK is transmitted to an access device, the UA waits to receive a retransmitted traffic data packet. When the HARQ NACK is received at an access device, the access device retransmits the incorrectly received packet to the UA. This process of transmitting, ACK/NACK and retransmitting continues until either the packet is correctly received or a maximum number of retransmissions has been reached.
In many cases it is desirable for an access device to transmit a large amount of data to a UA in a short amount of time. For example, a video cast may include large amounts of audio and video data that has to be transmitted to a UA over a short amount of time. As another instance, a UA may run several applications that all have to receive data packets from an access device essentially simultaneously so that the combined data transfer is extremely large.
One way to increase the amount of data that can be transmitted during a short period is to have an access device commence several (e.g., eight) data packet transmission processes in parallel. In HARQ scheme, there is a waiting time between the transmission of one packet and the reception of ACK/NACK response or between the reception of ACK/NACK and the retransmission. During this waiting time, the access device initiates another data packet transmission. To facilitate a HARQ scheme for each of a plurality of simultaneous packet transmissions, access devices and UAs are programmed to support parallel HARQ processes. To this end, each DCI formatted downlink resource grant on the PDCCH includes a three bit HARQ process number (HPN) or HARQ process identity/indicator (HPI) corresponding to an associated data packet. When a data packet is not correctly received, the incorrectly received packet and associated HPI are stored by the HARQ entity in a HARQ decoding buffer and a NACK is transmitted back to the access device to request retransmission of the data packet. The access device retransmits the data packet along with the HPI associated with the original transmitted data packet to the UA. When the retransmitted packet and HPI are received, the UA delivers the retransmitted packet to the HARQ process associated with the received HPI. The HARQ process attempts to decode the combined packet data and the HARQ process continues. Where the HPI is three bits, the maximum number of simultaneous HARQ processes is eight.
Another way to increase the rate of data transmission is to use multiple carriers (e.g., multiple frequencies) to communicate between an access device and UAs using carrier aggregation. Since the data transmissions in multiple carriers are operated independently, the number of separate HARQ processes required to manage additional data should also be increased. Currently there is one known way to increase the number of uniquely identifiable HARQ processes in a multiple carrier system. First, where multiple carriers are used, the UA HARQ entity can replicate the single carrier operation separate HARQ processes for each of the carrier frequencies in the usual fashion where the access device retransmits data packets using the same carrier as an original incorrectly received packet. For example, where the DCI format includes a three bit HPI and an access device and UA use four carriers, the access device and UA may facilitate eight separate HARQ processes for each of the four carriers for a total of thirty-two (i.e., 8N where N is the number of carriers) separate HARQ processes.
While multi-carrier systems are capable of facilitating large numbers of separate HARQ processes (e.g., forty in a case where five carriers and three bit HPIs are used), there are several reasons to limit the number of HARQ processes in at least some applications.
First, unfortunately, significant memory for HARQ buffer may be required to implement carrier aggregation (including, for example, soft bit combining where each received soft bit value is represented by several bits of memory associated with a set of HARQ processes), especially as the number of HARQ processes increases. Therefore, in some cases, limiting the total number of physical HARQ processes for UAs supporting carrier aggregation is desirable.
Second, using fewer than the maximum number of possible HARQ processes in a system enables a finer granularity when specifying a maximum achievable data transfer rate. In some cases, for example, it may be desirable to design UAs whose maximum data rate is equivalent to what could be carried on 1.5 or 2.5 carriers, in which case fewer total UA HARQ processes would be required. In contrast, where the maximum number of HARQ processes are supported, the maximum data rate granularity is in multiples of one carrier.
Third, as the number of carriers increases, trunking efficiency may also be expected to increase resulting in less need for the maximum number of UA HARQ processes. As the number of carriers increases, the percentage of time where the absolute maximum number of UA HARQ processes is required to support the peak achievable data rate (in a typically-loaded cell) decreases. Accordingly, there may be a less strict requirement, and it may be inefficient to support the maximum number of HARQ processes for multi-carrier implementations.
Where a multi-carrier system is to employ less than the maximum number of HARQ processes, there must be some way to map the three bit HPIs to specific HARQ process buffers in the UA.